The present invention relates to core drilling of rock or earth and in particular to an anti-blockage bearing for dislodging blockages of the core barrel.
Mechanical jars are known in apparatus for drilling in earth or rock. The purpose of the jar is to impart a force, generally a blow or series of blows, to dislodge a drill rod train which has become lodged in the drill hole.
Core drilling is widely used. In such drilling assemblies, the lower end of the drill string houses a cylindrical hollow member, called the core barrel, which serves to receive samples of the rock or earth for testing. Blockage of the core barrel would defeat the object of this form of sample drilling as no progress can occur and damage can result to the drill bit. Further, core retrieval with only partial core samples would substantially increase drilling cost.
Known jars have limitations. In general, drilling jars have a preset tensional threshold which, when exceeded, results in the delivery of a blow designed to dislodge the blockage. However, damage to the drill string can result if the tension control is improperly set. Such designs are generally not well suited to deliver blows in rapid succession and often deteriorate fairly rapidly due to the large frictional forces involved in their operation.